"In this area, the rebels control the countryside and open roads, and the Syrian army only controls the bases and the skies," he said on the telephone from an area north of the country's commercial capital. "In a 20-mile radius there are three bases, and the rest is controlled by rebels."

A Syrian army soldier who had defected to the rebels told NBC News that morale among the pro-Assad forces was very low.

The NBC News team in Syria did not release details of its exact location for safety reasons.

The news of rebel advances came after a newly formed joint command of rebel groups said it had chosen a former officer to head a new Islamist-dominated command. This was part of a Western-backed effort to put the opposition's house in order as President Bashar Assad's army takes hits that could usher his downfall.

The rebel groups chose Brigadier Selim Idris, one of hundreds of officers who have defected from Assad's army, as its head, opposition sources in Turkey said on Saturday.

Idris, whose home province of Homs has been at the forefront of the Sunni Muslim-led uprising, was elected by 30 military and civilian members of the joint military command after talks attended by Western and Arab security officials in the Turkish city of Antalya.

Nearly two years into the civil war in Syria, 41,000 people have been killed. Although the rebels have more control now, they fear chemical warfare. NBC's Richard Engel reports.

The unified command includes many with ties to the Muslim Brotherhood and to Salafists, who follow a puritanical interpretation of Islam. It excludes the most senior officers who have defected from Assad's military.

Rebel advancesOn the Damascus battlefront, Assad's forces used multiple rocket launchers on Saturday against several suburbs that have fallen to rebels who have fought their way to the edge of the city's international airport, where foreign carriers have suspended all flights.

Rebels, who have overrun several army bases near Damascus over the last month, appeared to be holding their ground, encircling a main military base in the northeastern suburb of Harasta, known as "idarat al markabat," near the main highway to Aleppo, according to opposition campaigners.

"The fighters made slight progress today. They captured a weapons depot and got to a tank repair facility in the base, but all 20 tanks inside were inoperational," Abu Ghazi, a rebel who was speaking from the area, told Reuters.

"The weather cleared and MiG fighters hit rebel positions around the base. Rocket launchers did not stop for the last three days. The site is crucial for the regime," he added.

Indeed, the state of the weather is key for rebels, according to NBC News' Balkiz.

"The rebels love it when it is overcast (because) aircraft can't bomb because they don't have the visibility," he said.